1. Field of the Invention
The present invention relates to improvements and modifications of an osmotically driven fluid dispenser and coating compositions for osmotically driven dispensers.
2. Description of the Related Art
Osmotically driven dispensers are described in commonly owned U.S. Pat. Nos. 3,760,984 and 4,320,758, which are hereby incorporated by reference in their entirety. The mini-osmotic pumps described in these patents as well as in the present invention are osmotically driven dispensers sized for use as therapeutic systems for administering drugs to animals and humans. The basic components of the pumps are an inner flexible bag that holds the drug charge; an intermediate layer of an osmotically effective solute composition, such as an inorganic salt, that encapsulates the bag; an outer shape-retaining membrane that is permeable to water and that encapsulates both the layer of osmotically effective solute composition and the encapsulated bag; and a filling/discharge port that communicates with the interior of the bag.
In operation, the bag is filled with drug via the filling/discharge port and placed in an aqueous environment, such as a body cavity or within body tissue. Water is imbibed from the environment by the osmotically effective solute through the membrane into the space between the inner flexible bag and the membrane. Since the bag is flexible and the membrane is rigid, the imbibed water enlarges the space between the membrane and the bag and exerts an inwardly squeezing pressure on the bag thereby displacing drug out of the bag through the filling/discharge port.
One shortcoming of the above-described pumps is the tendency to develop cracks in the region of the seal between the inner bag and the outer membrane. This tendency is pronounced when the outer membrane is formed from solvent-polymer mixtures in which the polymer is relatively less soluble. However, use of such solvents may be desirable due to safety and waste management concerns. Formation of fissures in the outer membrane in the region of the joint between the open end of the inner bag results in decreased performance or complete failure of the pump, evidenced by a decreased or non-constant flow rate or an absence of flow. The present invention is directed towards eliminating or reducing this shortcoming.